Sensitivity to iprodione and boscalid of Sclerotinia sclerotiorum isolates collected from rapeseed in China

Baseline sensitivity of Sclerotinia sclerotiorum, causal agent of stem rot of rapeseed, to a dicarboximide fungicide iprodione was determined using 50 isolates (historic population) collected in 2001 from the rapeseed fields without a previous history of dicarboximide usage. The 50% effective concentration (EC50) values to iprodione of these wild-type isolates ranged from 0.163 to 0.734 mu g/ml with a mean of 0.428 mu g/ml. In 2007 and 2008, 111 isolates (current population) were collected from rapeseed fields with 4-5 years of iprodione application. The EC50 values of these 111 isolates ranged from 0.117 to 0.634 mu g/ml. The historic and current populations were not significantly (P > 0.05) different in sensitivity to iprodione. The EC50 values of these 161 isolates to a carboxamide fungicide boscalid ranged from 0.002 to 0.391 mu g/ml with a mean of 0.042 mu g/ml. In the laboratory, three iprodione-resistant (IR) isolates HA17-IR, SZ31-IR, and SZ45-IR were induced from wild-type isolates HA17, SZ31, and SZ45, respectively. The EC50 values of the IR isolates were 200-fold higher than those of the original wild-type parents. The IR isolates showed an increase in osmotic sensitivity. The IR isolate HA17-IR lost its ability to produce sclerotia, and showed a significantly lower virulence on rapeseed leaves than its parent isolate HA17. In contrast, the IR isolate SZ31-IR had a significantly higher virulence than its wild-type parent SZ31. PCR assays showed that the partial two-component histidine kinase (os-1) gene, which is the putative target gene of iprodione, was deleted in the low virulent IR isolate HA17-IR. DNA sequence analysis showed that each of the other two IR isolates SZ31-IR and SZ45-IR had two point mutations in their partial os-1 genes. These results indicate that the mutations in os-1 gene may be associated with dicarboximide sensitivity, sclerotial development, and virulence in S. sclerotiorum. (C) 2009 Elsevier Inc. All rights reserved.